# Unusual dynamic charge-density-wave correlations in HgBa$_2$CuO$_{4+\delta}$

## TL;DR

This study uses high-resolution RIXS to reveal complex, energy-dependent charge-density-wave correlations in underdoped HgBa$_2$CuO$_{4+\delta}$, linking them to pseudogap phenomena and magnetic excitations.

## Contribution

First high-resolution RIXS measurement of dynamic CDW correlations in HgBa$_2$CuO$_{4+\delta}$, revealing multiple energy scales and their relation to pseudogap and magnetic excitations.

## Key findings

- Dynamic CDW correlations at ~40 meV above T_C
- Emergence of a high-energy (~160 meV) feature in charge response
- No significant coupling between CDW and magnetic excitations

## Abstract

The charge-density-wave (CDW) instability in the underdoped, pseudogap part of the cuprate phase diagram has been a major recent research focus, yet measurements of dynamic, energy-resolved CDW correlations are still in their infancy. We report a high-resolution resonant inelastic X-ray scattering (RIXS) study of the underdoped cuprate superconductor HgBa$_{2}$CuO$_{4+\delta}$ ($T_c = 70$ K). At $T=250$ K, above the CDW order temperature $T_\mathrm{CDW} \approx 200$ K, we observe significant dynamic CDW correlations at about 40 meV. This energy scale is comparable to both the superconducting gap and the previously reported low-energy pseudogap. At $T = T_c$, a strong elastic CDW peak appears, but the dynamic correlations around 40 meV remain virtually unchanged. In addition, we observe a new feature: dynamic correlations at significantly higher energy, with a characteristic scale of about 160 meV. A similar scale was previously identified in other experiments as a high-energy pseudogap. The existence of three distinct features in the charge response is highly unusual for a CDW system, and suggests that charge order in the cuprates is closely related to the pseudogap phenomenon and more complex than previously thought. We further observe the paramagnon dispersion along [1,0], across the two-dimensional CDW wavevector $\boldsymbol{q}_\mathrm{CDW}$, which is consistent with magnetic excitations measured by inelastic neutron scattering. Unlike for some other cuprates, our results point to the absence of a discernible coupling between CDW and magnetic excitations.